1. Field of the Invention
The invention relates to a method and a system for supplying a powder coating device. In particular, it relates to the series coating of workpieces such as, for example, car bodies, with powder lacquer.
2. Relevant Prior Art
Until now, powder lacquers have usually been aspired with the aid of a suction injector, which works on the Venturi principle, from a container fluidized with air and they are conveyed through plastic hoses in a powder-air mixture to the sprayer, in a process in which a small powder volume is conveyed in a large volume of air, in order to overcome the pressure drop in the conveyor hoses, although this leads to high flow velocities and the resulting tendency to form deposits in the conveyor hose. Any deposits have to be removed after the coating, at considerable effort by means which include emptying the hose by blowing, because any powder residues which later disengage, interfere with the coating and lead to errors in color during a color switch. “Color entrainments” to date cannot be entirely prevented because of the incompleteness of the cleaning of the hose. In addition, any powder residues removed during the cleaning of the hose are lost for the coating and have to be disposed of.
Additional undesired losses of powder are the result of imprecise dosage, with respect to quantity and/or switching times, of the coating powder supplied to the sprayer. Any powder which during the coating is sprayed as “overspray,” and which does not become deposited on the workpiece, can be collected in part and worked up for renewed use, however, from a technological and ecological point of view it is advantageous to keep the quantity of overspray as small as possible. This problem is also not solved satisfactorily by special, relatively expensive, dosage installations of known powder coating installations (EP 0 525 303, DE 199 37 425).
The difficulties in the cleaning of the powder hoses also have been one of the reasons why, at this time, no powder coating installations for car bodies, which would allow a rapid and frequent changing of paint color, are yet in use. To the extent that a change in powder color paint has been used in other branches of industry, one either had to use a corresponding large number of color specific lacquering booths, or one had to completely clean and refit the lacquering booth for each change in paint color (EP 0 200 681).
In the series coating of workpieces, such as car bodies, with a liquid lacquer and frequent changes of color, pigging systems have been used for some time for purposes including the reduction of lacquering and solvent losses (DE 197 09 988, DE 197 42 588, DE 100 33 986). For conveying powder lacquer, pigs so far could not be used without problem because the usual fluidization by means of an air stream in the direction of conveyance has not been possible because the pig blocked the line, and because of the high coefficients of friction, because of the tendency of the lacquer powder to deposit by sintering under pressure and to deposit during the conveyance, as well as because of the absence of compressibility.